A principle design goal associated with cargo aircraft is minimizing the time required to load and unload cargo containers to and from an aircraft's cargo compartment. Aircraft cargo normally is pre-packaged in or on specially designed containers or pallets known as unit load devices (hereinafter “ULDs”) that are sized and shaped to occupy substantially all available space within the cargo compartment of an aircraft. A modem aircraft's cargo compartment typically includes a cargo deck equipped with a plurality of ball mats positioned proximate to the aircraft's cargo door. The ball mats facilitate rolling movement of the ULDs through the door and into and out of the cargo compartment. Aircraft cargo decks also typically include roller trays that extend in fore and aft directions, and that facilitate rolling fore and aft movement of ULDs between the area proximate to the cargo door and various cargo stowage positions within the cargo compartment. In many modem cargo aircraft, ULDs can be loaded through the aircraft's nose section and through at least one side cargo door.
During loading and unloading operations, an aircraft's cargo deck may not be perfectly level, or can change from a level condition to a non-level condition as cargo is loaded or unloaded to and from the aircraft. In certain situations, an aircraft's cargo deck can slope in a direction that permits an unattended ULD to roll downhill toward the aircraft's open cargo door. In order to prevent an unattended ULD from rolling out of a cargo compartment through an open cargo door, cargo aircraft often include one or more blocking devices (hereinafter “rollout stops”) that can be incrementally spaced along the sill of the cargo door. In operation, such rollout stop devices act like one-way gates that permit intended movement of a ULD through an open cargo door and into an aircraft's cargo hold, and also prevent unintended rolling movement of a ULD out of the aircraft's cargo door.
Various types of rollout stops are known for blocking unwanted outward rolling movement of ULDs. Generally, such devices include at least one retractable guide head that can be selectively raised to a height above the cargo deck that is sufficient to block outward rolling movement of a ULD through a cargo door. Typically, such retractable guide heads have a generally upwardly and inwardly sloping top surface that is contacted by the leading lower edge and bottom surface of a ULD as the ULD passes over the stop and into a cargo hold, thereby forcing the upright guide head to automatically retract as the ULD passes over the guide head and into the cargo hold. Typically, the retractable guide heads are upwardly biased by one or more springs that maintain the guide heads in a raised position until the guide heads are forced to retract by an inwardly moving ULD. Once an inwardly moving ULD passes over the rollout stop, the springs return the guide heads to their raised positions, thereby generally preventing the just-loaded ULD or another already-loaded ULD from rolling in a reverse direction through the open cargo door. Accordingly, such rollout stops can be described as being “over-ridable” in an inward direction, and as being generally “non-overridable” in an outward direction. In order to permit ULDs to be unloaded from a cargo compartment without obstruction, the retractable guide heads generally are configured to be selectively latched or otherwise restrained in a lowered, non-blocking position when not in use.
Though prior art rollout stops can be effective to prevent outward rolling movement of most ULDs, the lower edges of a ULD can become warped from use. In some cases, if a lower trailing edge of a ULD is sufficiently upwardly bent, the lower trailing edge can be at an elevation that is sufficiently high to at least partially extend over a raised guide head as the ULD approaches the rollout stop in an outward direction. Accordingly, the warped lower trailing edge and bottom surface of the ULD may force the guide downward as the ULD passes over the stop in an outward direction, thereby undesirably overriding the rollout stop in the outward direction.
Another problem with prior art rollout stops is that they are not designed to yield to excessive contact loads with an outbound ULD. In situations where an outbound ULD approaches a prior art rollout stop at a high velocity and/or is especially heavy, the outbound ULD can impact the rollout stop with sufficiently high force to at least partially damage the aircraft structure to which the rollout stop is mounted.
Accordingly, there is a need for a rollout stop apparatus that substantially reduces the possibility that an outbound ULD will override the rollout stop. Preferably, the rollout stop should be relatively simple in construction such that production and repair costs are minimized. In addition, such a roll out stop should be designed to yield to an outbound ULD that contacts the rollout stop with sufficient force to otherwise damage the support structure of an aircraft to which the rollout stop is mounted.